Light emitting diode (LED) chips can generate visible or non-visible light in a specific region of the electromagnetic spectrum depending on the material composition of the LED. When it is desired to construct an LED light source that produces light of a color different from the output color of the LED, it is known to use a wavelength converter to convert all or a portion of the light output from the LED chip having a first wavelength or wavelength range (the “primary light” or “excitation light”) to light having a second wavelength or wavelength range (the “secondary light” or “emission light”).
Many wavelength converters include a wavelength converting composition that itself includes a polymeric matrix encapsulating a wavelength conversion material. The wavelength conversion material generally functions to convert primary light to secondary light via photoluminescence. In some instances the wavelength conversion material absorbs relatively high energy primary light, which can excite the wavelength conversion material to a higher energy state. When the wavelength conversion material returns to a lower energy state it emits secondary light, generally of a longer wavelength/wavelength range than the primary light. The wavelength/wavelength range of the secondary light can depend on the type of wavelength conversion material used. Secondary light of a desired wavelength/wavelength range may therefore be attained by proper selection of wavelength conversion material. This process may be understood as “wavelength down conversion.” An LED that is combined with a wavelength converter to produce secondary light may be described as a “wavelength converted LED.”
As noted previously a wavelength converting composition may include a matrix, which may include or be formed from a polymer such as silicone or an epoxy. The matrix (also referred to herein as an encapsulant) encapsulates a wavelength conversion material, such as phosphor particles. Configurations wherein a wavelength converter is disposed adjacent a surface of an LED chip are referred to herein as a “chip level conversion” or “CLC” configuration. In other configurations a wavelength converter may be placed remotely from the emitting surface of the LED. Such a configuration is referred to herein as a “remote conversion” configuration.
Quantum dots (QDs), also referred to as semiconductor nanocrystals, are relatively new materials that have potential use in the lighting industry. Like conventional phosphor particles, some quantum dots have the ability to absorb incident light (e.g., primary light) and reradiate light (e.g., secondary light) in another portion of the electromagnetic spectrum. QDs exhibit unique characteristics, which can be employed for creating wavelength converters with precisely designed output spectra. These unique properties include broad absorption spectrum (freedom of the choice of the excitation wavelength) and narrow-band emission (30-50 nm) with the peak wavelength determined by the material and size of the QD. Tunability of the peak emission wavelength of the QDs within few nanometers by changing their size offers a unique capability of creating finely tuned emission colors. As a result, quantum dots have been investigated for potential use in the formation of wavelength converters for semiconductor devices such as LEDs.